1. Field of the Invention
The present invention relates to a spatial optical transmission system for optically transmitting data in a free space. More specifically, it relates to a spatial optical transmission system of which directional full width at half maximum of the transmitted light is wide and which has a diffusion type transmission light source used for the communication distance of at most 10 m, a semiconductor laser module used for the spatial optical transmission system, and to an electronic appliance mounting the spatial optical transmission system or the semiconductor laser module.
2. Description of the Background Art
In a conventional spatial optical transmission system, a light emitting diode (LED) has been used as a light source for the transmission light. As a representative example, an optical transmitting/receiving device of IrDA (Infrared Data Association) shown in FIG. 21 has been known. The optical transmitting/receiving device includes a light emitting diode 112 having a peak wavelength of 850 to 900 nm and a light receiving element 102, and is applied to low speed data link of a short distance.
As a spatial optical transmission system using a semiconductor laser device for optical communication, a spatial optical link system for a space without humans in a normal state has been known. The semiconductor laser device is used in the spatial optical link system, as it has better condensing characteristic than the LED, and hence it is possible to transmit a light beam over a long distance while maintaining luminous flux of a small diameter. In a spatial optical transmission system disclosed in Japanese Patent Laying-Open No. 6-252856, radiation angle of a beam is limited narrow, to realize long distance and high-speed data link. In the spatial optical transmission system, transmission over a few to several kilometers is intended, and in order to suppress attenuation in the air, it is recommended to use a semiconductor laser beam having the wavelength range of 735 nm to 759 nm, 770 nm to 811 nm or 838 nm to 891 nm.
An LED is a non-coherent device and, generally, has a half maximum width of several tens nm. When the peak wavelength is 850 nm, for example, as shown in FIG. 22, the emission band expands to the range of about 750 to 1000 nm. Therefore, when a sharp optical filter is used in a transmitting/receiving device using an LED, the wavelength range that will be cut is wide, lowering signal light intensity. Thus, a sharp optical filter cannot be used. Further, when the LED is used, as the wavelength expands wide, it becomes necessary to expand effective sensitive wavelength band, in designing the light receiving element. When the effective sensitive wavelength band is made wider, however, background noise light such as sunlight or fluorescent lamp increases, so that signal to noise (SN) ratio degrades, and the minimum reception sensitivity lowers, that is, the maximum transmission distance becomes shorter.
When a light receiving element is molded by a resin containing black carbon, the background light on the shorter wavelength side is cut to some extent, and therefore, lower noise is expected. The wavelength of the LED as the transmission light source, however, is inherently wide, and therefore, significant noise reduction is not attained. Further, the color of the semiconductor laser module as a whole is limited to black, which is not very preferable considering the appearance of products such as mini disk players in silver, white or other colors.
In a system disclosed in Japanese Patent Laying-Open No. 6-252856 mentioned above, a semiconductor laser beam having the wavelength range of 735 nm to 759 nm, 770 nm to 811 nm or 838 nm to 891 nm is used, considering attenuation in the air. When radiation directional full width at half maximum is wide, however, light intensity attenuates abruptly as the transmission distance increases. Therefore, the usage is limited for short distance. In this case, it is unnecessary to consider attenuation by air, and oscillation wavelength of the light source is determined from the following different view points.
A light beam having a short wavelength of about 735 to 850 nm is much absorbed by a silicon light receiving element, and is absorbed at the surface of a pn or pin junction photodiode (PD), that is, on the light entering side. Therefore, it is necessary to make a layer on the light entering side efficiently thin. At this time, background light having wavelength shorter than the aforementioned laser beam wavelength is also absorbed at the surface layer, and as the surface layer is thin, it results in a diffusion current, and thus, noise component.
The above described use of the semiconductor laser beam as a light source is simply an application of a semiconductor laser device for spatial transmission, and, as compared with a conventional device using an LED, technical superiority is not very high.
In view of attaining higher technical superiority, it may be possible to use a steep optical filter for reducing background light, as the semiconductor laser device is used as the light source. Mounting of a steep optical filter, however, increases cost of the spatial transmission device, and narrows receivable angle of a receiver. Therefore, this option is industrially impractical.